Until now, ozone and chlorine have been widely used in water and sewage treatment. However, persistent substances that are not decomposed by ozone or chlorine may be contained in, for example, industrial wastewater, recycled water, and the like. In particular, removal of dioxins, dioxane, and the like is a major problem.
In some areas, a method of removing persistent substances by combining ozone (O3) with hydrogen peroxide (H2O2) or ultraviolet light, thereby causing hydroxyl radicals (OH radicals), which are higher in activity than ozone or chlorine, to be generated in water to be treated, is in practical use. However as equipment and operation costs are very high, this method is not very prevalent.
In view of this, a method has been proposed in which persistent substances are removed with high efficiency by causing OH radicals generated by a discharge to act directly on water to be treated. More specifically, a water treatment apparatus has been proposed in which a streamer discharge is formed by applying a pulse voltage between a linear high-voltage electrode and a cylindrical ground electrode that surrounds the high-voltage electrode, and water to be treated is treated by supplying the water to be treated in a water droplet state to a streamer discharge space from thereabove (see PTL 1, for example).
The water treatment apparatus in PTL 1 is provided with a gas suction supply means for sucking out gas from inside a treatment chamber and, during a preliminary process in which the water to be treated is formed into water droplets, supplying the sucked out gas into the water to be treated in the form of gas bubbles.
Therefore, with the water treatment apparatus in PTL 1, short-lived OH radicals generated in the streamer discharge space can be caused to act on the water to be treated efficiently. Further, ozone generated in the streamer discharge space can be used, without waste, for decomposition of a substance to be treated. As a result, decomposition treatment of the substance to be treated can be performed more efficiently.
In addition, a water treatment apparatus has been proposed in which a streamer discharge is formed by applying a pulse voltage between a linear high-voltage electrode and a cylindrical ground electrode surrounding the high-voltage electrode, and water to be treated is treated by supplying the water to be treated in a water droplet state into a streamer discharge space from thereabove, wherein grids constituted by an insulating material are formed in a plurality of stages in a region where the water droplets fall (see NPL 1, for example).
With the water treatment apparatus in NPL 1, short-lived OH radicals generated in the streamer discharge space can be caused to act on the water to be treated efficiently. Further, as the falling water droplets collide with the grids, falling speed thereof is lost, such that a residence time of the water droplets in the streamer discharge space can be prolonged. For this reason, a water treatment apparatus that realizes a higher decomposition efficiency and a higher decomposition speed can be obtained.